Radiation sensitive silver halide emulsions containing one or a combination of chloride, bromide and iodide ions have been long recognized to be useful in photography. Each halide ion selection is known to impart particular photographic advantages. Although known and used for many years for selected photographic applications, the more rapid developability and the ecological advantages of high chloride emulsions have provided an impetus for employing these emulsions over a broader range of photographic applications. As employed herein the term "high chloride emulsion" refers to a silver halide emulsion containing at least 50 mole percent chloride and less than 5 mole percent iodide, based on total silver.
During the 1980's a marked advance took place in silver halide photography based on the discovery that a wide range of photographic advantages, such as improved speed-granularity relationships, increased covering power both on an absolute basis and as a function of binder hardening, more rapid developability, increased thermal stability, increased separation of native and spectral sensitization imparted imaging speeds, and improved image sharpness in both mono- and multi-emulsion layer formats, can be realized by increasing the proportions of selected tabular grain populations in photographic emulsions.
The various photographic advantages were associated with achieving high aspect ratio tabular grain emulsions. As herein employed and as normally employed in the art, the term "high aspect ratio tabular grain emulsion" has been defined as a photographic emulsion in which tabular grains having a thickness of less than 0.3 .mu.m and an average aspect ratio of greater than 8:1 account for at least 50 percent of the total grain projected area of emulsion. Aspect ratio is the ratio of tabular grain effective circular diameter (ECD), divided by tabular grain thickness (t).
Although the art has succeeded in preparing high chloride tabular grain emulsions, the inclusion of high levels of chloride as opposed to bromide, alone or in combination with iodide, has been difficult. The basic reason is that tabular grains are produced by incorporating parallel twin planes in grains grown under conditions favoring {111} crystal faces. The most prominent feature of tabular grains are their parallel {111} major crystal faces.
To produce successfully a high chloride tabular grain emulsion two obstacles must be overcome. First, conditions must be found that incorporate parallel twin planes into the grains. Second, the strong propensity of silver chloride to produce {100} crystal faces must be overcome by finding conditions that favor the formation of {111} crystal faces.
Wey U.S. Pat. No. 4,399,215 produced the first silver chloride high aspect ratio (ECD/t&gt;8) tabular grain emulsion. An ammoniacal double-jet precipitation technique was employed. The tabularity of the emulsions was not high compared to contemporaneous silver bromide and bromoiodide tabular grain emulsions because the ammonia thickened the tabular grains. A further disadvantage was that significant reductions in tabularity occurred when bromide and/or iodide ions were included in the tabular grains.
Wey et al U.S. Pat. No. 4,414,306 developed a process for preparing silver chlorobromide emulsions containing up to 40 mole percent chloride based on total silver. This process of preparation has not been successfully extended to high chloride emulsions.
Maskasky U.S. Pat. No. 4,400,463 (hereinafter designated Maskasky I) developed a strategy for preparing a high chloride, high aspect ratio tabular grain emulsion capable of tolerating significant inclusions of the other halides. The strategy was to use a particularly selected synthetic polymeric peptizer in combination with a grain growth modifier having as its function to promote the formation of {111} crystal faces. Adsorbed aminoazaindenes, preferably adenine, and iodide ions were disclosed to be useful grain growth modifiers. The principal disadvantage of this approach has been the necessity of employing a synthetic peptizer as opposed to the gelatino-peptizers almost universally employed in photographic emulsions.
This work has stimulated further investigations of grain growth modifiers for preparing tabular grain high chloride emulsions, as illustrated by Takada et al U.S. Pat. No. 4,783,398, which employs heterocycles containing a divalent sulfur ring atom; Nishikawa et al U.S. Pat. No. 4,952,491, which employs spectral sensitizing dyes and divalent sulfur atom containing heterocycles and acyclic compounds; and Ishiguro et al U.S. Pat. No. 4,983,508, which employs organic bis-quaternary amine salts.
Maskasky U.S. Pat. No. 4,713,323 (hereinafter designated Maskasky II), continuing to use aminoazaindene growth modifiers, particularly adenine, discovered that tabular grain high chloride emulsions could be prepared by running silver salt into a dispersing medium containing at least a 0.5 molar concentration of chloride ion and an oxidized gelatino-peptizer. An oxidized gelatino-peptizer is a gelatino-peptizer treated with a strong oxidizing agent to modify by oxidation (and eliminate or reduce as such) the methionine content of the peptizer. Maskasky II taught to reduce the methionine content of the peptizer to a level of less than 30 micromoles per gram. King et al U.S. Pat. No. 4,942,120 is essentially cumulative, differing only in that methionine was modified by alkylation.
While Maskasky II overcame the synthetic peptizer disadvantage of Maskasky I, the requirement of a chloride ion concentration of at least 0.5 molar in the dispersing medium during precipitation presents disadvantages. At the elevated temperatures typically employed for emulsion precipitations using gelatino-peptizers, the high chloride ion concentrations corrode the stainless steel vessels used for the preparation of photographic emulsions. Additionally, the high chloride ion concentrations increase the amount of emulsion washing required after precipitation, and disposal of the increased levels of chloride ion represents increased consumption of materials and an increased ecological burden.
Tufano et al U.S. Pat. No. 4,804,621 disclosed a process for preparing high aspect ratio tabular grain high chloride emulsions in a gelatino-peptizer. Tufano et al taught that over a wide range of chloride ion concentrations ranging from pCl 0 to 3 (1 to 1.times.10.sup.-3 M) 4,6-diaminopyrimidines satisfying specific structural requirements were effective growth modifiers for producing high chloride tabular grain emulsions. Tufano et al specifically required that the following structural formula be satisfied: ##STR2## wherein Z is C or N; R.sub.1, R.sub.2 and R.sub.3, which may be the same or different, are H or alkyl of 1 to 5 carbon atoms; Z is C, R.sub.2 and R.sub.3 when taken together can be --CR.sub.4 .dbd.CR.sub.5 --or --CR.sub.4 .dbd.N--, wherein R.sub.4 and R.sub.5, which may be the same or different are H or alkyl of 1 to 5 carbon atoms, with the proviso that when R.sub.2 and R.sub.3 taken together form the --CR.sub.4 .dbd.N-- linkage, --CR.sub.4 .dbd. must be joined to Z. Tufano et al also contemplated salts of the formula compound. Tufano et al demonstrated the failure of adenine as a growth modifier. Thus, Tufano et al discourages the selection of heterocycles for use as grain growth modifiers that lack two primary or secondary amino ring substituents in the indicated relationship to the pyrimidine ring nitrogen atoms and those compounds that contain a nitrogen atom linked to the 5-position of the pyrimidine ring.